Hybrid electric vehicles (“HEVs”) utilize both an internal combustion engine and one or more electric machines (e.g., motors/generators) to generate power and torque. The electric motor/generator(s) within a hybrid electric vehicle provides the vehicle with additional degrees of freedom in delivering the driver-demanded torque and may also be used to control the output speed of the engine.
In one type of hybrid electric vehicle, commonly referred to as a “power split” type hybrid electric vehicle, the electric generator and the internal combustion engine are interconnected by use of a planetary gear set, and the electric generator selectively provides a reaction torque which may be used to control (e.g., to reduce and/or augment) the speed of the vehicle's engine. In this manner, the generator is used to control the speed of the engine and cooperates with the planetary gear set and a traction motor to provide a continuous variable transmission (“CVT”) effect. One drawback associated with this type of hybrid electric vehicle, arises from the limited amount of reaction torque which can be provided by the motor/generator.
Particularly, because the amount of torque produced by a typical internal combustion engine is relatively high, the reaction torque required to control the speed and/or output of the engine also must be relatively high. This presents a problem under certain vehicle operating conditions (e.g., high speed operating conditions), as the motor/generator is not always able to deliver the reaction torque desired or needed to control or slow the speed of the vehicle's engine. For example and without limitation, the graph 200 of FIG. 3 illustrates the performance of a typical hybrid electric vehicle generator over a range of speeds. In order to provide the relatively high reaction torque needed to control the speed/output of the engine, the size and capacity of the motor/generator used within the vehicle is typically increased, thereby undesirably adding to the vehicle's weight, expense, and the packaging space required to house the motor/generator. Alternatively, the capacity or output of the vehicle's engine may be decreased to allow the motor/generator to control the engine. However, decreasing the size of the engine undesirably results in a decrease of the overall power output of the hybrid electric vehicle.
There is therefore a need for a method and an apparatus for controlling the speed of an engine within a hybrid electric vehicle, which allows the size of the vehicle's motor/generator to be desirably reduced, or conversely, which allows the size of the vehicle's engine to be increased.